Ribosomal DNA loci derived from Brachypodium stacei are switched off for major parts of the life cycle of Brachypodium hybridum

Abstract

Nucleolar dominance (ND) is an epigenetic phenomenon that occurs in some plant and animal allopolyploids and hybrids, whereby only one ancestral set of 35S rRNA genes retains the ability to form the nucleolus, whilst the rDNA loci derived from the other progenitor are transcriptionally silenced. Since there is substantial evidence that ND is regulated developmentally, this study focusses upon the establishment and/or maintenance of ND during different stages of development in the model grass allotetraploid, Brachypodium hybridum. Fluorescence in situ hybridisation with a 25S rDNA probe to 3-D cells shows that ND is present not only in root meristematic and differentiated cells of this species, but is also present in male meiocytes at prophase I, tetrads of microspores and different embryonic tissues. The inactive state of B. stacei-originated rDNA loci is confirmed by silver staining method. We show that only B. distachyon-derived 35S rDNA loci form a nucleolus/nucleoli in the aforementioned tissues, whereas B. stacei-like loci remain highly condensed and thus transcriptionally suppressed. The establishment of ND during earlier stages of B. hybridum embryo development cannot be ruled out. However, our proposal of gradual pseudogenisation of B. stacei-like loci in the evolution of the allotetraploid seems to be more likely.